Question

What is echo? What should be the minimum distance between the observer and obstacle for echo to be heard?

Answer 1

Echo -

1. The sound returning back towards the source after suffering reflection from a distant obstacle (a wall, a hill, a row of buildings, etc.) is called an echo.
2. When the sound is reflected repeatedly from a number of obstacles, more than one echo, called multiple echoes are heard. Multiple echoes may be heard one after the other when the sound gets repeatedly reflected from distant high-rise buildings or hills. The rolling of thunder is an example of multiple echo formations.
3. The minimum distance (in air at 20⁰C) between the observer and the obstacle for the echo to be heard clearly should be 17.2 m.